This invention relates to horizon sensors for determining the position of the horizon,and more particularly to providing a second order radiance correction for such sensors to correct errors caused by radiance variations.
It is often necessary to determine the attitude of a satellite with respect to the earth. One means of doing this is to sense the position of the earth's horizon in three or more directions. In order to operate both in day and night, infrared sensors are usually used, which sense the warm earth against cold space. A simple horizon location concept is shown in FIG. 1. An infrared detector with a rectangular field of view is oriented toward and straddling the horizon as shown and is termed a static type sensor. The signal S from this detector will be proportional to X R, where X is the length of the detector field subtended by the earth and R is the radiance of the earth. If R is known, the position of the horizon with respect to the lower edge of the detector field can readily be determined by X=(k S)/R where k is a calibration constant including the width of the detector and its responsivity. For simplicity, it will be assumed that k is unity in the following discussion.
This simple design is generally inadequate because the earth radiance varies with geographic location and season. This problem can be overcome to a certain extent by using a pair of detectors, A and B as shown in FIG. 2. Detector A functions as described previously. Detector B has a smaller field located at the lower or earth edge of detector A, and its function is to continually view and measure the earth radiance level. This it can do, since its field will always be fully filled by the earth, whereas the signal from detector A involves both R and X.
The signal S.sub.B on detector B is given by bR. Thus R=S.sub.b /b and: ##EQU1##
This may be considered a first order radiance correction, since it assumes that the radiance is uniform over both detectors.
At lower altitudes, one must take into consideration the fact that the horizon is not sharp but diffuse because of the atmosphere. This results in a decreasing radiance with altitude above the horizon which is known as the horizon profile. It has been found that these profiles are most uniform in the 14-16 micron spectral region, which is a carbon dioxide absorption band. The reason for this is that carbon dioxide is uniformly mixed in the atmosphere and blocks any radiation from variable sources in the lower atmosphere such as clouds. However, even in this CO.sub.2 absorption band, the profiles do change with latitude and season.
FIG. 3 shows calculated profiles in January for a set of north latitudes. If a radiance-corrected horizon sensor of the type shown in FIG. 2 is used at an altitude where these profiles subtend an appreciable angle, and X is computed from Equation 1, significant errors will result.